Persistent phosphorescent materials are known and are commercially available. For example, metal sulfide pigments which contain various elemental activators, co-activators and compensators are available, including CaS:Bi, which emits violet blue light; CaSrS:Bi, which emits blue light; ZnS:Cu,Co which emits green light; and (ZnCd)S:Cu,Co which emits yellow or orange light.
Metal aluminate phosphorescent pigments, particularly alkaline earth aluminate oxides having the formula MAl2O4, where M is an alkaline earth metal or mixture of metals, are also available. These aluminate phosphorescent materials exhibit afterglow characteristics that last much longer in duration than do those of metal sulfide materials. Europium was incorporated into the strontium aluminate matrix which serves as an emitter. The afterglow became stronger if less SrO was used during the synthesis of SrAl2O4:Eu2+ resulting from formation of trapping centers associated with the Sr2+ vacancy.
The brightness and persistence time of SrAl2O4:Eu2+ was further improved by co-doping one or more co-activators into the strontium aluminate matrix using such elements as those of the Lanthanide series (e.g. lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium), tin, manganese, yttrium, or bismuth. The best result was obtained by co-doping Dy3+ with Eu2+ into SrAl2O4 and Nd3+ with Eu2+ into CaAl2O4 to get long persistent green and purple emission, respectively, see U.S. Pat. Nos. 5,424,006 and 5,686,022 both to Murayama.
In addition, persistent blue-green phosphorescent materials have also been disclosed wherein metallic cationic species have been substituted in the alkaline earth aluminate oxides having the formula MO.mAl2O3:Eu2+, R3+ wherein m is a number ranging from 1.6 to about 2.2, M is strontium or a combination of strontium and calcium and/or barium, Eu2+ is an activator, and R is one or more trivalent rare earth materials of the lanthanide series (e.g. lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium), yttrium or bismuth co-activators.
Cationic replacement of a portion of the Al3+ in the alkaline earth aluminum oxide matrix by divalent cations such as Mg2+ or Zn2+ has been reported in addition to replacing a portion of the alkaline earth metal ion (M2+) with a monovalent alkali metal ion such as Li+ or Na+.
Fluorides have been used in the manufacture of photoluminescent phosphors either as a flux material or to treat the surface of the phosphor crystals to provide improved moisture resistance as described in US Patent App. 2008/0277624A1.
As can be seen from the above examples, most work has been performed so as to improve the phosphorescence intensity and long-term persistence of the phosphors involving alterations in the cationic species, the amounts of the cations and ratios between them. Little attention has been paid to improvements in charging rates wherein the phosphor can be rapidly charged in low light environments and/or from weak activation sources. Also, very little investigation has been done on the replacement of the anionic, negatively charged oxygen ions in the phosphor matrix and the impact of such replacement on the luminescent properties of such a phosphor, such as, for example, intensity, persistence and charging rate.
Thus there is a need to investigate materials with improved charging rates as well as a need to investigate replacement of the anionic ions in phosphor matrices.